The presence of methane in subterranean coal deposits represents both a hazard and an economic benefit. In the mining of coal, the presence of poisonous methane gas is both a human health hazard and offers a high potential for explosion and fire. In the past, coalbed methane has been vented to the atmosphere and/or flared prior to the mining of the coal in order to reduce such hazards.
In recent times, it has been recognized that the venting of methane is both an economic waste and a large source of environmental pollution. The recovery of coalbed methane has thus become economically beneficial as a source of revenue as well as environmentally beneficial through a reduction in the amount of hydrocarbons released directly to the atmosphere. In this regard, it has become somewhat commonplace to recover methane from coal deposits in a manner substantially identical to normal natural gas production, i.e., drilling and completing a gas well into the coal seam and fracturing the well within the coal formation to enhance methane recovery.
One difficulty encountered with the recovery of methane from fractured or unfractured coal seams is that many coal reservoirs are water saturated. The bulk of the methane present in the coal seam is adsorbed onto the coal surface. For this reason, it is necessary to first remove a large portion of the water in the reservoir which results in a lowering of the reservoir pressure to a point whereby desorption of the methane from the coal surface occurs at a substantial rate. The relationship between pressure and desorption rate is known as the desorption isotherm. The desorption isotherm will vary from one coal to another. Because of this phenomenon, long periods of dewatering are generally required before methane production reaches a maximum rate.
The flow of water through the pores and fractures of a coal reservoir is significantly influenced by chemical adsorption of the water onto the surface of the coal. In most virgin, high-grade coal deposits, the coal surface is uncharged and the movement of water is restricted only by physical relationships. Physical restrictions result from either the viscosity of the flowing phase or the interfacial tension between the water and the coal. The presence of fines within a fracture, particularly at the fracture extremities, can result in restricted flow of the water into and within the fracture.
Fines are often observed during well production. Many of these fines are probably generated near the wellbore by mechanical failure of the coal. It is also likely that some of these fines originate from weathering of the coal seam in the vicinity of the fracture because of changes in the downhole environment resulting from stimulation, production and periodic workovers. Processes that lead to the development of fines include dehydration, oxidation, proppant embedment and changes in the in situ stresses within the coal seams. These fines tend to migrate within the fractures during the fracturing process and concentrate at the fracture tips restricting the flow of water in and through the fracture.
Air dissolved in the fluids which are introduced to the coal seams during the fracturing treatment and other well operations may also result in oxidation of the coal surface. In addition to generating fines, oxidation will also change the normally hydrocarbon-wet and electrically neutral surface of the coal to a surface that is water-wet and electro-negative, thereby inhibiting the dewatering process.
Wettability changes in the coal surface may also result from the choice and use of surfactants in stimulation or workover fluids. In a manner similar to weathering, surfactants may affect the coal surface and lead to changes in the relative permeability of the coal which can affect both the water drainage rate and fluid saturation level of the coal, either advantageously or detrimentally.
In an attempt to improve the dewatering of subterranean coal seams to effect the desorption and production of associated gas, a study has been made in order to identify surfactants which enhance the dewatering process while avoiding damaging the surface characteristics and relative permeability of the coal reservoir. Likely candidates for selection of a beneficial surfactant are the class of chemicals used as dewatering or filtering aids in the process of beneficiation of mined minerals such as metallic ores and coal. These processes typically involve the grinding of the mineral-containing material and flocculation and flotation so that the desired minerals are concentrated in a water slurry which is then filtered to remove the water.
Surfactants which have proven beneficial in the removal of water from filter cakes in mineral beneficiation processes include polyoxyethylene ethers of a hexitol anhydride partial long-chain fatty acid ester such as described in U.S. Pat. No. 2,864,765; oxyalkylated surfactants such as described in U.S. Pat. No. 3,194,758, 4,156,649 and 4,206,063. Other beneficial dewatering surfactants include dialkylsulphosuccinates such as described in U.S. Pat. No. 4,097,390 and 4,146,473. While use of these filtration and dewatering aids is suggested by their beneficial properties in dewatering of slurried fines under atmospheric pressure conditions, their use in a pressurized, porous subterranean environment has met with considerably less success.
One material which has met with limited success in subterranean use is a fluorinated alkyl quaternary ammonium iodide surfactant system such as described in U.S. Pat. No. 4,028,257. While this material has shown beneficial results in the initial dewatering of coal seams, the dewatering benefits gradually decline over time apparently due to leaching of the surfactant from the coal surface due to the passage of fluids through the pores and fractures to the wellbore.